A recent blurb on CFL.ca says its Andrew Harris. I'm a long time Wpg BB fan, but sorry this pinnacle is reserved for Russ Jackson.
Harris is great but not the "greatest ever". He's right up there with Jon Cornish, Joe Krol, Red Story, Normie Kwong, Ronnie Stewart, Don Getty, etc.
Unfortunately, unless the CFL changes its rules which exempt 3 quarterbacks from the overall import limit, there will never be another Canadian QB to be #1 for his career and lead his team to Grey Cup championship. This ill advised rule means that teams have zero incentive to develop a Canadian QB.
